This doubles as a bug report, I think, but after trying to get whatever they give you for killing them both for a few hours I'm just really curious as to what I'm going through all this trouble to get. So firstly, I was wondering about that.
Secondly, I did manage to kill both of them just now with no reward. I used Weytwut to put Yin and Yang right by each other, killed one with fire, absorbed it's blood pool with blood mage and stood in its spot while I killed the other. Got a "One alive: false" message... and then nothing happened. I noticed a gold necklace at the bottom of the screen after I finished exploring but that seems like a pretty lackluster reward for all the trouble it takes to accomplish this.
On one final random side note, haven't found Dracul yet. Where does he like to hide?